dimwit

from WordNet (r) 3.0 (2006)

dimwit
    n 1: a stupid incompetent person [syn: {dimwit}, {nitwit},
         {half-wit}, {doofus}]

from Moby Thesaurus II by Grady Ward, 1.0

30 Moby Thesaurus words for "dimwit":
      Boeotian, block, clod, cluck, dolt, donkey, dope, dullard, dumb,
      dumb cluck, dumbbell, dummy, dunce, gowk, jobbernowl, lackwit,
      lamebrain, lightweight, looby, loon, niais, nincompoop, ninny,
      ninnyhammer, nitwit, noddy, put, stupid, thickwit, witling